1. Field of the Invention
The present invention relates to an optical disc apparatus for recording or reproducing a signal on an optical disc using near-field light and an optical disc drive control method employed in the optical disc apparatus, and, more particularly, to a technique for controlling a gap between an optical disc and a lens.
2. Description of the Related Art
In recent years, in order to increase the recording density of an optical disc for recording or reproducing a signal using laser light, optical disc apparatuses for recoding or reproducing a signal using near-field light have been proposed. In such an optical disc apparatus using near-field light, laser light is emitted onto an optical disc using a Solid Immersion Lens (SIL). It is necessary to set the gap between the optical disc and an end surface of the SIL disposed in an optical pickup provided with an objective lens unit etc. to a distance (near field) at which near-field light is generated. The distance is generally half the wavelength of input laser light, and is a very short distance, for example, a several tens of nanometers. The distance at which near-field light is generated is referred to as a near field, while the distance at which near-field light is not generated is referred to as a far field.
In optical disc apparatuses in the related art such as DVDs for recording or reproducing a signal in a far-field region, the above-described gap control problem does not arise. However, in optical disc apparatuses using near-field light, since the distance between an optical disc and an SIL is very short, the gap control is very important. That is, since the distance between the optical disc and the SIL is very short, the SIL may collide with the optical disc under the influence of small change such as vibration externally transmitted to a disc apparatus or disc surface runout. This leads to damage to both of the SIL and the optical disc.
Thus, optical disc apparatuses using near-field light are easily affected by vibration externally transmitted thereto, shock externally given thereto, dust on an optical disc, a defect in the optical disc, etc. Methods of avoiding these effects include a method of improving robustness against disturbance by widening the servo band of a focus control system to approximately 13 kHz and a method of improving robustness against disturbance in principle by employing a conical-shaped lens.
In addition to the above-described methods performed in a signal processing system, there is a method of preventing disc damage caused when a lens collides with the surface of an optical disc by covering the surface of the optical disc with a hard coat (topcoat). This method is used for DVDs etc. For example, an example in which the method is used for an optical disc using a near field is described in T. Ishimoto et al. Proc. The 19th symp. on Phase Change Optical Information Storage, p. 27 (2007).